This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/JP98/03382 which has an International filing date of Jul. 29, 1998, which designated the United States of America.
This invention relates to a process for purifying crystalline substances containing impurities and, more particularly, to a process for purifying crystalline substances by crystallization which comprises feeding crystalline substances containing impurities in the state of solid to a continuous crystallizer.
A process known for the purification of crystalline substances comprises forming a solution or a melt of crystalline substances containing impurities with or without addition of a solvent and then cooling the solution or the melt to precipitate the object crystals. Where the crystallization is performed continuously, the precipitated crystals are melted again by a heating device provided at the bottom of a continuous crystallizer and discharged as a melt to recover the purified crystals. A crystallization process of this type is distinguished by its large throughput and its capability of attaining a high degree of purity with relative ease.
Japan Tokkyo Koho Sho 58-46322 (1983) describes a purification process which uses an agitated continuous crystallizer of the tower type containing a cooling zone in the upper section and a purifying zone in the lower section and effects the purification by charging the feed midway between the cooling and purifying zones, precipitating the crystals by cooling the feed slowly in the cooling zone while agitating only in the same plane with generation of substantially no force to transfer crystals in the vertical direction, bringing the crystal slurry of enhanced purity into countercurrent contact with the mother liquor in the purifying zone for further enhancement of the purity of the crystals, melting the crystals by a heating device provided beneath the purifying zone, and recovering the crystals as a melt. This purification process can be executed in an extremely simple apparatus with high productivity and it is highly practical when applied to an apparatus of relatively small size to medium size. However, it is difficult or impossible to purify certain kinds of crystalline substances by this process, which limits the scope of application of this process.
Japan Tokkyo Koho Sho 63-36802 (1988) describes a purification process which utilizes a tower type continuous crystallizer similar to the one described in the aforementioned Japan Tokkyo Koho Sho 58-46322 (1983) and effects the purification by melting the crystalline substances containing impurities, cooling, solidifying and pulverizing in advance and charging the powders and particles thus obtained to the crystallizer. This process has an advantage in that it is easy to prepare the feed crystals to be charged to the continuous crystallizer, but it became clear that the same problems as above remained unsolved.
In a process for purification by continuous crystallization wherein the feed consisting of crystalline substances containing impurities is charged as solid, an object of this invention is to provide a process for purification by crystallization which is applicable to a relatively wide range of feeds and operable continuously at high efficiency.
In a process for purifying crystalline: substances which comprises charging the feed crystals containing impurities in a solid state to a tower-type continuous crystallizer equipped with an agitating device from the top, purifying the crystals by bringing them into countercurrent contact with the melt formed by melting the precipitated crystals by a heating device provided in the lower section, taking out the purified crystals from the lower section as a melt, and taking out the mother liquor containing the concentrated impurities from the upper section, this invention relates to a process for purifying crystalline substances which comprises maintaining the temperature of the mother liquor existing in the upper section in a specified range by heating the feed crystals or the upper section of the tower-type continuous crystallizer or making the average particle diameter of the feed crystals 2.5 mm or more.
This invention also relates to a process for purifying crystalline substances which comprises heating the feed crystals to a point in the range between (normal temperature +10xc2x0 C.) and (the melting point of the feed xe2x88x9210xc2x0 C.) and charging the heated feed crystals to the aforementioned tower-type continuous crystallizer from the top.
This invention further relates to a process for purifying crystalline substances which comprises providing a heating device in the upper section of the aforementioned tower-type continuous crystallizer and heating the mother liquor existing in the upper section.
This invention still further relates to a process for purifying the aforementioned crystalline substances which comprises making the average particle diameter of the feed crystals 2.5 mm or more and forming the feed crystals in the shape of flakes with an average thickness of 0.3 mm or more. A preferred feed is benzoic acid.
A continuous crystallizer equipped with an agitator useful for this invention is satisfactorily a crystallizer of the vertical tower type provided with an inlet for the feed and the outlet for the mother liquor in the upper section and a heating device and an outlet for the product in the lower section. Although a variety of apparatuses may be used, those described in Japan Tokkyo Koho Sho 58-46322 (1983) and,xe2x80x9cAromatics,xe2x80x9d Vol. 37, pp. 109-127 (1985) or their partial modifications are desirable. In particular, the above-cited apparatuses from which the cooling device in the upper section is removed or in which the cooling device is replaced with a heating device are desirable.